Demand for thinner electronic devices is growing in recent years, and there is a need for making internal components, especially printed wiring boards on which electronic components are mounted, even thinner. Since the thickness of a printed wiring board on which electronic components are mounted (hereinafter referred to as “A” for the sake of convenience) is given as a sum of the thickness of the printed wiring board itself (hereinafter referred to as “B” for the sake of convenience) and height of the components (hereinafter referred to as “C” for the sake of convenience) (A=B+C), the aforementioned need can be met by reducing B or C or both. However, how much B (thickness of printed wiring board) and C (height of components) can be reduced is limited, and the industry has been waiting for breakthrough measures.
With regard to this point, Patent Literature 1 (hereinafter referred to as the “background art”) listed below describes a technology to form cavity areas on a printed wiring board and mount components in these cavity areas. If the depth of the cavity area is D, for example, this background art provides the same effect as reducing the height of components C by D, and effectively reduces, to a substantial degree, the thickness of component-mounted printed wiring board A.
With a printed wiring board conforming to the aforementioned background art, however, the “bottom” of cavity areas for mounting electronic components is not very strong and, when electronic components are mounted in the cavity areas, these cavity areas for mounting components will crack due to the pressure applied to the component surface if a strong, flat jig is not placed below the insulation layer forming the bottom, or if the height of components C exceeds the depth of cavity area D. In the worst case scenario, the bottom may come off. This problem can also occur when the height of components C is less than the depth of cavity area D.
FIG. 18 shows the structure of the background art. In this figure, a printed wiring board 1 is constituted by a metal sheet 2 having, on one side of it, a resin film 3 and insulation sheet 4 attached on top of each other, as well as an electronic component 6 mounted in a cavity area 5 formed on this metal sheet 2. Here, A is the thickness of the printed wiring board 1 on which the component is mounted, B is the thickness of the printed wiring board 1 itself, C is the height of the component 6, and D is the depth of the cavity area 5. Here, the magnitude correlation “C>D” holds true, meaning that a part of the component 6 is projecting from the cavity area 5.
In this condition, with the electronic component mounted, an unwanted pressure P may be applied to the surface of the component 6 when the printed wiring board 1 is assembled into an electronic device. A similar pressure P may also be applied, even after the printed wiring board 1 has been assembled, to the surface of the component 6 via an enclosure of the electronic device.
The mechanical strength of the printed wiring board 1 is primarily assured by the metal sheet 2, but the strength of the location where this metal sheet 2 is missing, or specifically a bottom 5a of the cavity area 5, depends on the strength of the resin film 3 and insulation sheet 4 that are much more fragile than the metal sheet 2, and consequently the bottom 5a of this cavity area 5 may detach depending on the degree of the aforementioned pressure P.
As for printed wiring boards having cavity areas for mounting components, there is a need in the market, of late, for ultra-thin boards that were not before required, such as boards of 1 mm or less in thickness. With these ultra-thin printed wiring boards, the aforementioned problem of mechanical fragility becomes more serious. If the insulation layer at the bottom of the cavity area is only several tenths of a millimeter thick, the bottom may crack or detach even with a very small force.
When printed wiring boards having cavity areas for mounting components began being available on the market, these boards were much thicker than 1 mm. Accordingly, forming the cavity area by machining the board, for example, was fairly easy. On the other hand, to form a cavity area on an ultra-thin printed board whose thickness is only 1 mm or even less, first and foremost it is necessary to overcome the aforementioned problem (mechanical fragility at the bottom of the cavity area), because unless this problem is overcome, the above market need of late cannot be met.
For example, current technology is sufficient to form a cavity area of 0.4 mm in depth on a board of 0.5 mm in thickness. A module can be made thinner by the depth of this cavity area. This thickness reduction of only 0.4 mm or so is enough to meet the market need of late. Rather, a primary reason why this market need cannot be met is the aforementioned mechanical fragility at the bottom of the concaved area.
Patent Literature 1 Japanese Patent Laid-open No. Sho 55-145390